How Diodes Work -- The Learning Circuit 23

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Diodes and Bipolar Junction Transistors (BJTs) - Measuring the Temperature Coefficient

Semiconductors typically have four valence electrons in their outer shell.  Silicon, being a semiconductor, also has four outer valence electrons.  This outer shell can hold up to eight electrons. Electrons are most stable when their outer valence shell has eight electrons, a rule known as the octet rule. Each silicon atom shares an electron to its neighboring silicon atom in order to satisfy the octet rule.  When silicon atoms form covalent bonds they crystallize into a very strong structure known as a crystal or a lattice.

In the P-type region silicon is doped with boron or gallium.  Boron and Silicon have only three outer electrons.  When mixed to a silicon lattice, they form “holes” in the crystal structure electron has nothing to bond to.  The absence of electrons gives it a positive charge.  In the N-type region silicon is doped with antimony, phosphorus, or arsenic. The fifth electron becomes a free electron.  It is free to go wherever the current takes it.  These free electrons are negative charge carriers.

The point where the N-Region and the P-Region meet is called the PN Junction. Near the junction the positive charges and the negative charges, having opposing charges are drawn to each other like magnets. The free electrons in the N-type region migrate over and fill the holes in the P-type region.  Because of the charged particles moving around, the area near the junction in the P-type region becomes slightly negatively charged while the area near the junction in the N-type region becomes slightly positively charged.  This area is known as the depletion zone. Eventually, the depletion zone becomes charged enough to stop electron migration. In a silicon diode this happens at around .7 Volts.